U.S. Pat. No. 4,496,455 discloses a sensor having an internal oxygen reference which operates according to the diffusion limit current principle disclosed in U.S. Pat. No. 4,356,065 (German patent publication 2,711,880).
This sensor includes a solid electrolyte made of zirconium dioxide. The solid electrolyte carries two pairs of electrodes permeable to oxygen and these electrodes are comprised, in part, of platinum.
One of the two electrode pairs in combination with the electrolyte defines the measuring cell. One of the two electrodes operates as a cathode and is indirectly exposed to the gas to be measured (measured gas) via a porous intermediate layer operating as a diffusion limiter. Oxygen molecules diffuse out of the gas to be measured to the cathode and are there catalytically split up into oxygen ions. The negative oxygen ions migrate to the anode under the influence of a voltage applied externally to the electrodes. In accordance with the limit-current principle, the voltage is so dimensioned that the intensity of the electric current carried by the oxygen ions is essentially dependent upon the rate at which the oxygen diffuses out of the gas to be measured to the measuring electrode. This rate is known to be dependent upon the oxygen concentration in the exhaust gas so that the diffusion limited current flow defines a measure for the composition of the measured gas.
The anode adjoins a small porous filled volume in order to maintain a constant oxygen partial pressure at the anode of the measuring cell. This constant oxygen partial pressure is desirable for measurement reasons and the volume is substantially insulated from the measured gas and from the ambient air.
Likewise, the anode of a second electrode pair borders on this volume. This second electrode pair together with the electrolyte and an externally applied voltage define a pump cell.
The cathode of the pump cell is exposed directly to the gas to be measured so that the pumped ion current is not limited by a diffusion boundary. The oxygen particles exiting at the anode of the pump cell form an internal reference atmosphere within the small porous volume.
Fine bores are provided to the gas to be measured in order to limit the pressure of this reference atmosphere to permissible values.
The above-mentioned patent publication provides no suggestion as to how or pursuant to what criteria the bores are to be dimensioned which act as a means to compensate for overpressure.
In this context, the following problem is presented. On the one hand, the pressure-limiting connection of the reference atmosphere to the measured gas must guarantee that no mechanical damage occurs to the sensor even for a short-term increased pump current. The connection must therefore make a certain minimum particle current possible.
If, on the other hand, the connection permits a particle current which is too great, then the reference atmosphere can become adulterated by the influence of the measured gas.
The measured gas can, for example, be the exhaust gas of an internal combustion engine. If this is the case, uncombusted fuel particles can diffuse out of the exhaust gas and into the reference gas volume. There, the fuel particles can bind oxygen and thereby considerably affect the oxygen partial pressure. This case can especially occur for a mixture composition rich in fuel or for an engine at standstill and with the pump voltage switched off. One conceivable remedy comprises increasing the pump current during operation of the engine so high that the oxygen particle current from the reference gas volume provides for a scavenging of the reference gas volume via the connection to the measured gas.
This advantage is offset in that a long-term increase of the pump current can lead to a decomposition of the electrolyte and therefore to a destruction of the sensor.
These problems can also occur when the overpressure compensation is not directed to the measured gas but is instead directed to the ambient air because the immediate ambient of the sensor can be exposed to fuel vapor or other impurities especially when the engine is at standstill.